Generally speaking, a cardiac pacemaker is an electrical device used to supplant some or all of an abnormal heart's natural pacing function, by delivering appropriately timed electrical stimulation signals designed to cause the myocardium of the heart to contract or "beat". Stimulation signals usually have well-defined amplitude and pulse width characteristics which can be adjusted to meet physiologic and device power conservation needs.
The strength (amplitude) and duration (pulse width) of the stimulation signals must be of such magnitude that capture is maintained, to prevent serious complications and even death. Yet, it is desirable for these magnitudes not to be higher than is needed for a reasonable safety margin for longer battery life. Chief among the problems is that stimulation signal thresholds necessary for maintaining capture often fluctuate in the short term, and gradually change in the long term. It has been clinically observed that the lowest threshold is observed immediately after implantation of the pacemaker (the acute threshold). Inflammation in the tissue around the tip of the stimulation electrode requires greater energy to propagate the stimulation signals, thereby driving the threshold up sharply during the first two to six weeks to its highest level (the peak threshold). Some of the inflammation reduces over the long-term, to lower the threshold below the peak level--the chronic threshold. However, the chronic threshold does not reduce to the acute level, since some permanent fibrous tissue, requiring greater energy than non-fibrous tissue for signal propagation, remains around the electrode tip. In the short-term, thresholds may decrease with exercise, for example, and may increase with various activities, including sleep.
Some prior art implantable pulse generators (IPGs) which serve as cardiac pacemakers have an automatic capture feature to maintain capture without the need for clinical or patient intervention. These IPGs typically rely upon electrical sensors similar to pacing leads (consisting of insulated conducting wire, electrode tips and a connector for connecting the lead to the IPG) to sense the presence of capture in response to the stimulation signals. The function and accuracy of the these sensors have been adversely affected by one or more of factors including, but not limited to: myopotentials (electrical signals which are the product of muscle movement); stray electromagnetic interference (EMI); problems with the sensor sensitivity (either too sensitive or not sensitive enough); and variations of the sensed electrical signals as a result of changes in thoracic pressure (for example, due to respiration, coughing or sneezing).